Strongly Luminescent Tetradentate Palladium(II)‐TADF Emitters. Blue TADF And TADF‐Sensitized OLEDs with External Quantum Efficiencies Over 23%
Man‐Ki Sit, Glenna So Ming Tong, Tsz‐Lung Lam, Gang Cheng, Faan‐Fung Hung, Kwok‐Ming So, Lili Du, Kin‐On Choy, Kam‐Hung Low, Chi‐Ming Che
Abstract
Abstract Due to the low‐lying Pd(4d)‐orbitals, luminescent palladium(II) complexes usually show 3 IL phosphorescence (IL = intraligand) with excited state lifetimes in the range of hundreds of microseconds, which limits their application in blue organic light‐emitting diodes (OLEDs). Here, a molecular design strategy for the development of strongly luminescent Pd(II)‐based thermally activated delayed fluorescence (TADF) emitters with fast radiative decay rate constants and reverse intersystem crossing rate ( k RISC ) constants is presented. The synthesized tetradentate [Pd(N*C*C^N)] TADF emitters show sky‐blue to yellow (λ max = 484–565 nm) emission with excited state lifetimes in the range of 0.9−8.7 µs and high photoluminescence quantum yield of up to 77% in CH 2 Cl 2 solution. In 5 wt.% 2,8‐bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) thin films, these [Pd(N*C*C^N)] TADF complexes display blue to sky‐blue (476−496 nm) emission with quantum yields close to unity. Variable‐temperature emission lifetime measurements, femtosecond time‐resolved spectroscopy, and density functional theory (DFT) and time‐dependent DFT (TDDFT) calculations together reveal ultrafast k RISC rate constants of these [Pd(N*C*C^N)] TADF emitters. Using these Pd(II)‐TADF emitters as dopants, sky‐blue TADF and blue TADF‐sensitized OLEDs with small efficiency roll‐offs achieved high maximum external quantum efficiencies (EQEs) of 24.8% and 23.1%, respectively.